Not Applicable
This invention relates to an image display systems, and more particularly, to a shortened asymmetrical light integrating tunnel that improves the uniformity and brightness of images produced by color video projection displays.
Projection systems have been used for many years to project motion pictures and still photographs onto screens for viewing. More recently, presentations using multimedia projection systems have become popular for conducting sales demonstrations, business meetings, and classroom instruction.
In a common operating mode, multimedia projection systems receive analog video signals from a personal computer (xe2x80x9cPCxe2x80x9d). The video signals may represent still, partial-, or full-motion display images of a type rendered by the PC. The analog video signals are typically converted in the projection system into digital video signals that control a digitally driven light valve, such as a liquid crystal display (xe2x80x9cLCDxe2x80x9d) or a digital micro mirror device (xe2x80x9cDMDxe2x80x9d).
A popular type of multimedia projection system employs a light source and optical path components upstream and downstream of the image-forming device to project the image onto a display screen. An example of a DMD-based multimedia projector is the model LP420 manufactured by In Focus Systems, Inc., of Wilsonville, Oreg., the assignee of this application.
Significant effort has been invested into developing compact portable projectors producing bright, high-quality, color images. However, the optical performance of such projectors is often less than satisfactory. For example, projected images having suitable uniform brightness are difficult to achieve, especially when using compact portable color projectors in a well-lighted room.
Because LCD displays have significant light attenuation and triple path color light paths are heavy and bulky, portable multimedia projectors typically employ DMD displays in a single light path configuration. Producing a projected color image with this configuration typically requires employing some form of sequential color modulator, such as a color shutter, color-switchable light source, multiple light-emitting diode arrays, or a color wheel, to generate a frame sequential color image. Unfortunately, such color modulators often produce minimal light intensity and/or have significant light attenuation.
The use of color wheels in frame sequential color (xe2x80x9cFSCxe2x80x9d) display systems has been known for many years and was made famous (or infamous) in early attempts to develop color television sets. With technological advances, however, color wheel display implementations are still useful because of their simplicity, color purity, and inherent image convergence.
FIG. 1 shows a typical prior art FSC display system 10 in which a sensor 12 senses a timing mark 14 to detect a predetermined color index position of a motor 16 that rotates a color wheel 18 having respective red, green, and blue filter segments R, G, and B. A light source 20 projects a light beam 22 through color wheel 18 and a relay lens 24 onto a display device 26, such as an LCD-based light valve. A display controller (not shown) drives display device 26 with sequential red, green, and blue image data that are timed to coincide with the propagation of light beam 22 through the respective filter segments R, G, and B of color wheel 18.
FIG. 2, shows a prior art multimedia projector 30 capable of projecting images having increased, uniform brightness relative to the LCD-based display of FIG. 1. A light source 32 emits polychromatic light that propagates along an optical path 34 through projector 30. Light source 32 generates intense light by employing a metal halide arc lamp 36 and an elliptical reflector 38. Optical path 34 includes a condenser lens 40, a color wheel 42, a rectangular light integrating tunnel 44, a relay lens 48, a DMD 50, and a projection lens 52. The optical components are held together by an optical frame 54 that is enclosed within a projector housing (not shown). A display controller 56 receives color image data from a PC 58 and processes the image data into frame sequential red, green, and blue image data, sequential frames of which are conveyed to DMD 50 in proper synchronism with the angular position of color wheel 42. A power supply 60 is electrically connected to light source 32 and display controller 56 and also powers a cooling fan 62 and a free running DC motor 64 that rotates color wheel 42. Display controller 56 controls DMD 50 such that light propagating from relay lens 48 is selectively reflected by DMD pixel mirrors either toward projection lens 52 or toward a light-absorbing surface 66 mounted on or near optical frame 54.
DC motor 64 rotates color wheel 42 at about 6,650 to 7,500 rpm. Color wheel 42 includes color filter segments R, G, and B that each surround about 120 degrees of color wheel 42. Color wheel synchronization is achieved by optically detecting which color filter segment is in optical path 34 and for how long. Particular colors of light propagating through color wheel 42 are sensed by a color selective light sensor 68 to generate synchronization timing data. Light sensor 68 is positioned off optical path 34 to receive scattered light.
To increase projected image brightness uniformity, a rectangular input aperture 70 of light integrating tunnel 44 collects a majority of the light exiting color wheel 42 and homogenizes the light during propagation through tunnel 44 to a rectangular output aperture 72. Light exiting output aperture 72 is focused by relay lens 48 onto DMD 50. However, because DMD 50 is tilted obliquely to optical path 34, the image of output aperture 72 on DMD 50 is Keystone distorted, causing image overfill at the far end of DMD 50 resulting in light loss, reduced brightness, and brightness nonuniformity across DMD 50.
Conventional light integrating tunnels typically have rectangular input and output apertures and may be formed as either an air-tunnel with reflective inside surfaces or may be a solid optical material, such as glass, quartz, or plastic with polished outer surfaces. For air tunnels, the light is reflected off the reflective inside surfaces, and for solid tunnels the light is totally internally reflected off the polished outer surfaces. Because the input and output apertures are rectangular, the four tunnel walls are perpendicular at their abutting edges. To achieve a suitable degree of uniformity, such light integrating tunnels have a length that is at least five times the transverse diagonal dimension of the tunnel.
There are other previously known light integrating tunnel designs that compensate for various light path-related illumination nonuniformities. For example, U.S. Pat. No. 5,303,084 for LASER LIGHT BEAM HOMOGENIZER AND IMAGING LIDAR SYSTEM INCORPORATING SAME describes a rectangular light integrator tunnel having tapered curved recesses extending between its input and output apertures for adjusting the uniformity of the light beam exiting the integrator. In another example, U.S. Pat. No. 5,844,588 for DMD MODULATED CONTINUOUS WAVE LIGHT SOURCE FOR XEROGRAPHIC PRINTER describes a wedge-shaped light integrator tunnel for providing anamorphic illumination of a wide aspect ratio DMD. However, neither conventional integrating tunnels nor either patent addresses the above-described excess overfill problem caused by oblique illumination of a DMD. The tunnels described are also physically large.
What is needed, therefore, is a compact way of capturing as much of the light propagated through a color modulator as possible and uniformly imaging the light on an obliquely positioned reflective light valve without light overfill.
An object of this invention is, therefore, to provide a compact apparatus and a method for capturing as much of the light propagated through a color modulator as possible and uniformly imaging the light on an obliquely positioned reflective light valve without light overfill.
Another object of this invention is to provide an integrator tunnel having an input aperture shaped to optimally collect light propagated through a color modulator.
A further object of this invention is to provide an integrator tunnel having an output aperture shaped and/or angled to optimally image homogenized light onto an obliquely positioned reflective light valve.
Still another object of this invention is to provide an integrator tunnel that combines the above-described objects into a single light integrator tunnel that provides suitable light integration and is shorter than a conventional integrator tunnel having substantially the same amount of light integration.
In a first embodiment of this invention, a multimedia projector includes a source of polychromatic light that propagates through color filter segments of a color modulator, such as a color wheel, and enters a rectangular input aperture of an asymmetrical light integrating tunnel that spatially integrates the light into a spatially uniform pattern as it exits a nonrectangular output aperture of the tunnel. The uniform illumination exiting the nonrectangular output aperture is re-imaged by a relay lens onto a reflective light valve that is mounted obliquely to the longitudinal axis of the tunnel.
An advantage of the light tunnel of this invention is that the image of the nonrectangular output aperture on the light valve compensates for any Keystone distortion, illumination overfill regions, and illumination drop-off regions, thereby preventing light loss, increasing brightness, and brightness uniformity across the light valve.
In another embodiment of this invention, the asymmetrical light integrating tunnel further includes a trapezoidal input aperture that further improves illumination brightness of the multimedia projector by solving the following problem. When a color wheel is employed, the filter segments are separated by areas referred to as xe2x80x9cspokesxe2x80x9d that traverse the input aperture such that portions of different colored filter segments are both propagating light into the light integrating tunnel at the same time. To prevent this condition from causing color impurities or loss of brightness in the projected image, a display controller xe2x80x9cturns offxe2x80x9d the light valve during time periods when the spokes are traversing the input aperture. The trapezoidal input aperture allows the light valve to be turned off for a substantially shorter than conventional time period because the spokes traverse a smaller timing angle that is bounded by the sloping edges of the trapezoidal input aperture.
An advantage of the trapezoidal input aperture is that the resulting smaller timing angle allows a longer light valve on time such that less light is lost and projected image brightness is improved. This is particularly advantageous when the light source beam spot significantly overfills the input aperture.
Another advantage of either of the above asymmetrical light integrating tunnels is that it provides suitable light output uniformity in a length that is shorter than a conventional rectangular light integrating tunnel.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof that proceed with reference to the accompanying drawings.